Hybrid powertrain systems generate propulsion torque from two or more energy sources, e.g., hydrocarbon-based fuels through an internal combustion engine, and electric power through one or more electric machines, with the propulsion torque transferred through a torque transmission device to an output member coupled to a driveline. Such powertrain systems may employ either or both a charge-depleting mode and a charge-sustaining mode, wherein such strategies refer to managing a state of charge (SOC) of an energy storage system for the electric machines. In a charge-depleting mode, a powertrain control system controls powertrain operation such that the SOC of the energy storage system achieves a minimum state during a vehicle trip, or key-on cycle, and manages torque from the internal combustion engine and torque from the electric machine(s) based thereon. In a charge-sustaining mode, a powertrain control system controls powertrain operation such that the SOC of the energy storage system is at the same level at the end of a vehicle trip as it was at the beginning of the vehicle trip, and manages torque from the internal combustion engine and torque from the electric machine(s) based thereon.
Control systems for operating hybrid powertrain systems control torque outputs of the engine and electric machine(s) and apply torque transfer elements in the transmission to transfer torque in response to operator-commanded output torque requests, taking into account fuel economy, emissions, drivability, and other factors. A control system monitors various inputs from the vehicle and the operator and provides operational control of the hybrid powertrain, including controlling transmission operating mode and gear shifting, controlling torque outputs from the engine and electric machine(s), and regulating the electrical power interchange among the electrical energy storage device and the electric machines to manage outputs of the transmission, including torque and rotational speed.